The present invention relates to a liquid crystal display, more particularly to a homeotropic alignment liquid crystal display of active matrix type.
Generally, the In-plane Switching(hereinafter IPS) mode liquid crystal display has been suggested to improve narrow viewing angle characteristics of the Twisted Nematic (hereinafter TN) mode liquid crystal display. However, the IPS mode liquid crystal display makes an electric field parallel to substrates thereby improving the viewing angle characteristics. The IPS mode liquid crystal display has also a slow response time and a complicated manufacturing process is required to make a dual-domain. U.S. patent application Ser. No. 09/050,292 discloses a vertically aligned IPS (hereinafter IPS-VA) mode liquid crystal display to solve the foregoing problems.
The conventional IPS-VA mode liquid crystal display has following constitution.
Referring to FIGS. 1A and 1B, a counter electrode 51a and a pixel electrode 51b are disposed parallel to each other at an inner surface of a lower substrate 50 in a stripe manner. A first homeotropic alignment layer 53 is formed over the lower substrate 50 where the counter and pixel electrodes 51a, 51b are formed.
A color filter(not shown) is formed at an inner surface of an upper substrate 55 and a second homeotropic alignment layer 56 is formed on a surface of the color filter.
A liquid crystal layer 57 having a plurality of liquid crystal molecules 57a is interposed between the lower substrate 50 and the upper substrate 55. Herein, the liquid crystal molecules 57a are nematic molecules and have both dielectric and refractive anisotropic properties.
A first polarizing plate 58 is disposed at an outer surface of the lower substrate 50 and a second polarizing plate 59 is disposed at an outer surface of the upper substrate 55. Polarizing axes of the first and second polarizing plates 58,59 are crossed each other, and they make an angle of approximately xc2x145xc2x0 with an electric field being generated between the counter electrode 51a and the pixel electrode 51b. A phase compensating film 60 is interposed between the upper substrate 55 and the second polarizing plate 59 so as to compensate the refractive anisotropy of liquid crystal molecules 57a. 
In the absence of electric field between the counter electrode 51a and the pixel electrode 51b, as shown in FIG. 1A, the liquid crystal molecules 57a are arranged such that their long axes are disposed almost perpendicular to the substrates 50,55 according to the influence of the first and second homeotropic alignment layers 53,56. Consequently, since an incident light to pass the first polarizing plate 58 does not change its polarizing state while passing the liquid crystal molecules 57a, the incident light does not pass the second polarizing plate 59. The screen according to viewing angles shows a complete dark state since the refractive anisotropy of the liquid crystal molecules 57a according to the viewing angles is compensated by the phase compensating film 60.
On the other hand, a predetermined voltage is applied between the counter electrode 51a and the pixel electrode 51b, as shown in FIG. 1B, an electric field E is formed almost parallel to the lower substrate 50. And then, the liquid crystal molecules 57a which are disposed perpendicular to the substrates, are arranged such that their long axes are arranged to be parallel to the electric field E in case a liquid crystal of positive dielectric anisotropy is used. At this time, liquid crystal molecules 57a in the left side of the center of the electric field E are aligned in a clockwise direction and liquid crystal molecules 57a in the right side of the center of the electric field E are aligned in a counter-clockwise direction thereby forming two domains without incurring additional manufacturing process. Herein, liquid crystal molecules 57b in the center of the electric field E are affected by both sided liquid crystal molecules 57a having the same intensity in opposite directions. Therefore, they maintain their initial vertically aligned state. At this time, the liquid crystal molecules 57b keeping their initial vertically aligned state become a boundary of the two domains.
As described, since the liquid crystal molecules 57a are arranged parallel to the electric field E, an incident light to pass the first polarizing plate 58 changes its polarizing state while passing the liquid crystal layer 57. Accordingly, the light can pass the second polarizing plate 59, whereby the screen can be shown in a white state.
The IPS-VA mode liquid crystal display as constituted above improves the response time characteristics of display devices since the devices employ homeotropic alignment layers, and there is no need to perform any rubbing process to form two domains.
However, the IPS-VA mode liquid crystal display incurs following problems.
Since an electric field in one direction is formed within a pixel, the viewing angle characteristic of a region where the electric field is formed is superior to that of other regions where no electric field is formed.
In other words, as shown in FIG. 2, liquid crystal molecules in a horizontal direction where the electric field is formed, i.e. at 0xc2x0 and 180xc2x0 make a complete symmetry in their left side and right side since the liquid crystal molecules are arranged symmetrical themselves. However, liquid crystal molecules in perpendicular and oblique directions where no electric field is formed, i.e. at 90xc2x0, 270xc2x0 and at 45xc2x0, 135xc2x0, do not make any symmetry. Especially, in the oblique directions, there is even occurred a color shift that a selected color is shown in the white state screen.
Accordingly, it is one object of the present invention to provide a homeotropic alignment liquid crystal display having a multi-domain capable of obtaining a complete viewing angle characteristic at all azimuth angles in the screen.
It is another object of the present invention to provide a homeotropic alignment liquid crystal display having fast response time.
It is still object of the present invention to provide a homeotropic alignment liquid crystal display capable of preventing color shift in all azimuth angles in the screen.
So as to accomplish the forgoing objects, the present invention provides a homeotropic alignment liquid crystal display having a multi-domain comprising:
an upper substrate and a lower substrate opposed each other to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a first electrode formed on the inner surface of the lower substrate; and
a second electrode formed on the inner surface of the upper substrate, wherein the first electrode and the second electrode form an electric field for driving the liquid crystal molecules;
wherein in the absence of electric field between the first and second electrodes, the liquid crystal molecules are aligned such that their long axes are vertical to surfaces of the substrates,
wherein after applying a selected voltage to the first and second electrodes, first and second electric fields are formed simultaneously, the first electric field having a selected angle with a first direction and the second electric field having a symmetry with the first electric field with respect to the first direction;
wherein the selected angle between the first electric field and the first direction is in the range of 20xcx9c70xc2x0.
The present invention further provides a homeotropic alignment liquid crystal display having a multi-domain comprising:
an upper substrate and a lower substrate opposed each other to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and both defining pixel regions;
a counter electrode formed at each pixel region in the lower substrate, the counter electrode including a body of a rectangular frame shape, a first branch being disposed parallel to the gate bus line and simultaneously connecting lengthwise sides of the body and dividing a region surrounded by the body into a first space and a second space, a second branch diverged from the body or the first branch toward the first space in the form of a diagonal line, and a third branch diverged from the body or the first branch toward the second space in the form of a diagonal line;
a pixel electrode formed at each pixel region in the lower substrate and the pixel electrode forming an electric field together with the counter electrode, the pixel electrode including a first bar overlapped with one of the lengthwise sides of the body of the counter electrode and disposed parallel to the data bus line, a second bar diverged from the first bar and extended to be overlapped with the first branch of the counter electrode, a third bar diverged from the first or second bar and interposed between the second branches of the counter electrode in parallel, and the fourth bar diverged from the first bar or the second bar and interposed between the third branches of the counter electrode in parallel;
a switching means formed adjacent to an intersection of the gate bus line and the data bus line for transmitting a signal from the data bus line to the pixel electrode; and
homeotropic alignment layers interposed between the lower substrate and the liquid crystal layer and between the upper substrate and the liquid crystal layer, wherein the counter electrode and the pixel electrode are formed in the lower substrate;
wherein the second branch makes an angle xcex8 with the second branch of the counter electrode, the third branch makes an angle xe2x88x92xcex8 with the second branch of the counter electrode;
wherein the angle xcex8 is in the range of 20xcx9c70xc2x0.
Herein, a first polarizing plate is disposed at an outer surface of the lower substrate and a second polarizing plate is disposed at an outer surface of the upper substrate. A polarizing axis of the first polarizing plate is coincided with the first direction or the second direction and a polarizing axis of the second polarizing plate is arranged perpendicular to the polarizing axis of the first polarizing plate.